The present invention relates to elastomeric films, and more particularly to elastomeric optical interference films in which the reflected wavelength of light impinging on the film changes as the film is reversibly deformed.
Coextruded multilayer films have been made which comprise multiple alternating layers of two polymers with individual layer thicknesses of 100 nanometers or less. Such multilayer films are described, for example, in Alfrey et al, U.S. Pat. No. 3,711,176. When polymers are selected to have a sufficient mismatch in refractive indices, these multilayer films cause constructive interference of light. This results in the film transmitting certain wavelengths of light through the film while reflecting other wavelengths.
The reflection and transmission spectra for a particular film are primarily dependent on the optical thickness of the individual layers, where optical thickness is defined as the product of the actual thickness of the layer times its refractive index. Films can be designed to reflect infrared, visible, or ultraviolet wavelengths of light depending on the optical thickness of the layers. The films produced in accordance with the above mentioned Alfrey patent exhibit iridescence and changing colors as the angle of incident light on the film is changed.
The Alfrey patent also describes an elastomeric film made up of many alternating layers of two polymers. As the film is stretched, the change in the thickness of the layers causes the film to reflect different wavelengths of light. As the film is relaxed, the layers return to their original thickness and reflect their original wavelengths.
A problem with the elastomers described by Alfrey was that they had only a small refractive index mismatch, which limited the optical interference effects which could be achieved. Additionally, such elastomers had less than ideal transparency, where not readily coextruded, and tended to contain gels due to partial cross-linking. Accordingly, there still exists a need in the art for improved elastomeric optical interference films which are not subject to the drawbacks of prior art films.